1. Field of the Invention
The present invention relates generally to an automatic brightness limiting (ABL) circuit for preventing the color dispersion of a displayed picture, which is caused by an excessive beam current flowing through a CRT (Cathode-Ray Tube). The circuit may be used, for instance, in a picture tube of a television receiver. More particularly, the present invention relates to an improvement to a response characteristic of such an automatic brightness limiting circuit.
2. Description of the Prior Art
In a conventional prior art automatic brightness limiting circuit as shown in FIG. 1, a DC voltage B+ is supplied to one terminal of the primary coil of a flyback transformer FBT and horizontal synchronization pulses HS from a horizontal output section 11 of a conventional horizontal deflection circuit is applied to the other terminal of the primary coil of the flyback transformer FBT. Then, a high voltage is induced at the secondary coil of the flyback transformer FBT. As a result, a high voltage output HV for driving a color picture tube, a focus control voltage FV and a screen control voltage SV are drawn from a conventionl high voltage output section 10 which includes the flyback transformer FBT.
In such a conventional automatic brightness limiting circuit, if an input image signal to a color output section 13 is increased, a beam current flowing in the color picture tube is also increased. At this time, the variation of the beam current is detected at a coupling point between resistors R.sub.1 and R.sub.2, which is coupled to the lower terminal of the secondary coil of the flyback transformer FBT. Specifically, the current flowing through the resistor R.sub.1 which is connected to a DC voltage source B.sup.+ is divided and supplied to the lower terminal of the secondary coil of the flyback transformer FBT and the resistor R.sub.2. Then, the current supplied to the resistor R.sub.2 is applied to a brightness adjusting section 12 through smoothing capacitors C.sub.1 and C.sub.2 and a rectifying diode D.sub.1. The output voltage from the brightness adjusting section 12 is, in turn, supplied to the control input port of the color output section 13 as a control voltage.
When the beam current flowing in the color picture tube is increased, the voltage drop across the resistor R.sub.1 is also increased. This voltage drop causes the input voltage to the brightness adjusting section 12 to be decreased. Then, the beam current can also be decreased, since the amount of the input image signal to the color output section 13 is reduced.
However, in such a prior art automatic brightness limiting circuit, there is a problem because the response characteristic of the circuit is very slow due to large constant values of the smoothing capacitors C.sub.1 and C.sub.2. Therefore, such a prior art circuit is inapplicable to the CRT apparatus which require a high speed response characteristics and deteriorates the resolution of a displayed picture.